Most of electric and electronic appliances generate heat while they are used, so that the removal of generated heat therefrom is necessary for their normal operation. With the intention of removing the generated heat from those appliances, many means have been proposed. For instance, in miniature electronic parts, especially electronic devices provided with integrated circuits, thermally conductive materials, such as a thermally conductive grease and a thermally conductive sheet [Japanese Tokkai Sho 56-28264 (U.S. Pat. No. 4,265,775) and Japanese Tokkai Sho 61-157587, wherein the term "Tokkai" means an "unexamined published patent application], have been used as such means.
In general, such an electronic device comprises integrated circuits and cap parts for protecting them, and a thermally conductive material is applied so as to contact directly with both of the circuit element and the heat-reducing part, or indirectly therewith via certain materials. Thus, the heat generated from integrated circuit chips during operation is transmitted in the thermally conductive material to be transferred directly or indirectly to the heat-reducing part, and further radiated therefrom. The rough sectional view of such an electronic device, wherein a thermally conductive material is used, is shown in FIG. 1.
As the aforementioned thermally conductive material, a heat-reducing grease of the type which uses a silicone oil as a base material and a zinc oxide or alumina powder as a thickener is already known [Japanese Tokko Sho 52-33272 and Japanese Tokko Sho 59-52195 (U.S. Pat. No. 3,885,984), wherein the term "Tokko" means an "examined patent publication"). In recent years, aluminum nitride has been developed as a thickener which enables further improvement of thermal conductivity (as disclosed, e.g., in Japanese Tokkai Sho 52-125506).
Since aluminum nitride has insufficient oil-holding power, however, its content in a silicone oil is limited to a very narrow range, specifically the order of 50-95 parts by weight per 100 parts by weight of silicone oil. Accordingly, the contribution of aluminum nitride to improvement in the thermal conductivity of silicone grease is not great although the aluminum nitride itself has a good thermal conductivity.
In addition, Japanese Tokkai Sho 56-28264 discloses the thixotropic thermally conductive material comprising a liquid organosilicone carrier, silica fibers in an amount required for effectively preventing the liquid carrier from exuding, and at least one thickener selected from among dendrite-form zinc oxide, flake-shaped aluminum nitride and flake-shaped boron nitride. In this thermally conductive material also, the enhanced oil-holding power is secured by incorporating spherical silica fibers as an essential component; as a result, the content of an aluminum nitride powder is inevitably lowered. Therefore, a sufficient improvement in thermal conductivity cannot be expected in this case also.
The drawback as described above can be mitigated by combining a particular organopolysiloxane with spherical grains of hexagonal aluminum nitride to enable the incorporation of a very large amount of aluminum nitride in the silicone oil (Japanese Tokkai Hei 2-153995).
When the aluminum nitride grains are coarse in size, however, gaps are formed among the grains because aluminum nitride is a very hard material having Mohs's hardness of 7-9. Accordingly, even if the aluminum nitride powder is added in an increased amount with the intention of heightening the thermal conductivity, the thermal conductivity attained is below expectation, namely of the order of 5.5.times.10.sup.-3 cal/cm.multidot.sec.multidot..degree. C., and so a satisfactory result has not been achieved as yet.
As a means to solve the above problem, a method of using a fine aluminum nitride powder and a coarse aluminum nitride powder in combination can be adopted [Japanese Tokkai Hei 3-14873 (U.S. Pat. No. 5,011,870)]. In this case, however, the grease prepared, although the thermal conductivity thereof is improved, is low in consistency (in other words, hard) and poor in dispensation suitability. Therefore, such a means is undesirable from a practical point of view.
Thus, a method of using an organopolysiloxane of the kind which can hold a large amount of metallic powder in combination with at least one metallic powder selected from the group consisting of ZnO, Al.sub.2 O.sub.3, AlN and Si.sub.3 N.sub.4 was also proposed (e.g., in Japanese Tokkai Hei 2-212556 and Japanese Tokkai Hei 3-162493). However, satisfactory heat-reducing grease is not yet realized by such combinations.